1. Field of the Invention
This invention relates to washing systems wherein liquid is passed through a porous mat disposed on a screen over a vacuum head.
2. General Discussion of the Background
Various industrial processes require that a mass of porous material be washed in order to remove chemical or other impurities. For example, this need appears in the sugar industry, where sugar is washed from bagasse; in the textile industry where excess dyes are washed from fabric; in mining where impurities are washed from ore; and in the paper industry.
In a standard paper production line, wood chips are cooked with chemicals in aqueous solution, the precise composition of the cooking chemicals depending on the particular process and desired paper product. This step, normally carried out in a digester under heat and pressure, breaks down the wood by dissolving the organic compounds that hold the cellulose fibers together.
The mixture of pulp, spent cooking chemicals, and organic materials, collectively known as stock, is then fed to a series of washers. The most common type of washing system includes a rotary vacuum drum onto which the stock is spread as a mat. The drum has a cylindrical, porous outer surface, most commonly a screen. A negative pressure is maintained inside the drum, such that liquid in the mat is pulled into the interior of the drum and thereby separated from the pulp. A shower, which is disposed above the mat and extends axially along the drum, directs relatively clean liquid at and through the pulp mat to wash out chemical substances, dirt and organic solids. Typically in the brown stock area, there are three drums in sequence with wash liquid flowing from drum to drum countercurrently to the direction of the pulp movement. Each drum can have multiple showers to direct wash liquid at its pulp mat.
The final effluent from the drum washing operation is black liquor containing water, spent cooking chemicals, dirt and organic materials. Such liquor typically contains approximately 15% solid material, which must be separated from the water to allow reuse of the inorganic pulping chemicals in the liquor. Separation of the water and solids also reduces environmental problems when disposing of the liquor.
The solids and water are typically separated by an evaporation process in which the liquor passes through a series of evaporators. Within the evaporators, steam moves countercurrent to the liquor flow until the liquor is concentrated to a 60% solids content, at which point the liquor is burned in a boiler. The solid organic materials provide the fuel to generate steam for the evaporators, and inorganic chemicals smelt out the bottom of the boiler to be reused. The steam from the liquor recovery part of the cycle supplies most of the mill's steam needs.
It is apparent that the more dilute the liquor, the more energy must be expended in evaporating the water to recover the solids. At the same time, it is necessary to efficiently remove the chemicals from the pulp to provide a satisfactorily clean pulp. Thoroughly washing the mat improves the efficiency of chemical removal, but the large quantity of water typically required for thorough washing with existing showers forms a dilute liquor which requires a high expenditure of energy to separate water and solids.
One existing pulp mat shower is a weir constructed such that water flows between a pair of plates down onto and through the mat. Another type of shower is a whistle sprayer, which propels a stream of water under pressure towards the mat.
Yet another prior shower includes a shower pipe having a pair of diffusion flanges depending from the underside of the pipe. Each diffusion flange has a parabolic, concavely curved diffusion surface over which water flows from spray holes in the pipe to direct the water tangentially to the rotary drum. The diffusion flanges do not contact the surface of the mat being washed.
Existing showers which direct a stream of washing liquid towards a rotating mat are often unable to evenly distribute water across the drum. The mat is therefore washed unevenly across its width. Even the most efficient spray type showers give a less than uniform washing because the pulp mat is not of uniform thickness. Moreover, with spray type showers, some of the washing liquid does not move through the mat, but instead rolls down the drum and into the pulp vat, which increases water consumption, evaporation costs, and biological oxygen demand (BOD), while decreasing overall washing effectiveness.
Some of these problems were addressed by the Uniflow shower which is sold by Comarco Industrial, Inc., of Beaverton, OR. This shower, which is disclosed in U.S. Pat. Nos. 4,205,541 and 4,616,489, includes an elongated chamber placed axially above the drum. The lower portion of the chamber is narrow and structured such that a head of liquid builds up within the chamber to create a static liquid pressure which forces liquid to leave the chamber through a slot along its bottom. The head within the chamber, in combination with the vacuum inside the drum, causes liquid to flow out of the slot and through the pulp mat. Additionally, a pair of resilient skirts are attached to the slot and extend to the mat to guide the water perpendicularly to the mat and form somewhat of a seal between the mat and skirts, thus helping force water through the mat. Also, depending on the location at which the Uniflow shower is mounted, water can collect between one of the flexible skirts and the mat surface to form a standing puddle that extends across the width of the mat to facilitate even washing.
Although the Uniflow shower increases efficiency and evenness of washing by allowing a standing body of water to form on the surface of the mat, other problems may be created by the skirts contacting the mat surface. The Uniflow skirts are resilient, but they may disturb the pulp mat if the pulp mat is delicate due to low pulp consistency or density. Moreover, clumps on the surface of the mat may sometimes be knocked back into the vat. Preferably, all clumps should be smoothed into the surface of the mat.
The extremely corrosive chemicals used to make paper present another problem. Any device for washing a pulp mat must be constructed to withstand the rigors of this corrosive environment. Elastomeric skirts which are flexible enough to accommodate widely varying mat heights and pulp clumps are subject to chemical attack and must be frequently replaced.
It is accordingly an object of the present invention to provide a shower with an assembly for removing chemicals from a mat without disturbing the pulp mat.
Another object is to provide such an assembly which can be retrofitted to existing showers.
It is yet another object of the invention to avoid improper or random distribution of washing fluid over the width of the pulp mat.
Yet another object of the invention is to decrease water consumption, evaporation costs, and biological oxygen demand in a paper washing process.
Even yet another object of the invention is to provide a shower apparatus which is capable of accommodating varying mat heights and clumps of material on the mat, while being able to endure exposure to corrosive chemicals.
Also, it is an object of the invention to provide such a shower which automatically compensates for varying pulp mat thicknesses or clumps of pulp riding on the pulp mat.